New version of Receiver Protection Unit


Chris Moulding
 

We have released a new version of the Receiver Protection Unit so that the maximum output is below the strict 10 dBm limit for SDRPlay receivers.

In the previous version the maximum output level was 17 dBm which is OK for the majority of receivers but was still too high for SDRPlay receivers.

The protection unit uses a range of methods such as a gas discharge tube, isolation transformer and cascaded diode limiters to bring excessive RF power down to a safe level.

If connected to a ground it will also drain static electricity from the antenna and a 10 kA gas discharge tube will help with nearby lightning strikes.

With the latest version you could connect a 150 W transmitter for that ham radio "Oops!" moment without exceeding the SDRPlay receiver limit.

More details on the web page: http://www.crosscountrywireless.net/receiver_protection_unit.htm

Regards,

Chris


Chris Moulding
 

OK it's official.

SDRPlay have approved the VLF/HF Receiver Protection Unit as recommended for use below 50 MHz with SDRPlay RSP receivers.

I've added the change to the web page:

http://www.crosscountrywireless.net/receiver_protection_unit.htm

Regards,

Chris


Roger Need
 

The factor not often mentioned in the specs for receiver protection boxes is the OIP2 and OIP3.  In order to do the limiting they all contain some type of non-linear limiting device (diode, saturating transformer etc.) and these generate unwanted spurious signals.  This happens at signal levels well below the maximum output limit spec of 10 to 20 dBm.

The OIP2 and OIP3 two carrier tests will give an indication of what to expect when you use one of these protection devices.  Perhaps Chris will let us know what he measures on his unit.


Roger


Mark Lucas
 

Chris,

I have the older version VLF/HF Receiver Protection Unit between the base unit of a VLF/LF/HF Active Vertical Antenna and the HF Multicoupler.  I have fallen behind in my installation of a transceiver but hope to finish this spring. You ran tests for me and all appears safe using 50 to 100 watts from VHF & UHF.  I plan on upgrading my license to General for some HF use.  I wasn't aware of the +10dBm max on my RSPdx.  Would it be beneficial to upgrade to the new version receiver protection unit or are there any mods I can make to the old ?

Both your HF active vertical and the Broadband Active Antenna are working great.  I have them through your multicouplers directly to the RSPdx and other receivers and scanners.  I even use an MFJ-1026 Noise Canceller to adjust the phase between the two antennas with outstanding results.

Thanks.

Mark


Chris Moulding
 

Mark,

The earlier version of the Receiver Protection unit wasn't designed with the SDRPlay receivers in mind.

The RF power allowed through in the early version is higher than the +10 dBm limit of the SDRPlay receivers.

The current version of the protection unit will correctly limit the output to below +10 dBm even if tested with 150 W output from an Icom HF marine transceiver directly into the protection unit input for a short time (10 seconds).

I don't recommend trying to modify an existing unit as we test each unit with 150 W before dispatch.

It's good to hear that your antennas are working well. One of my long term projects is to start making the Broadband Active Antenna again but redesigned to be powered directly from the bias tee in a SDRPlay receiver. Once I'm happy with the over-voltage protection circuit I'll build a batch of them.

Regards,

Chris


Mark Lucas
 

Ok.  I'll order the new one when I'm closer to finishing up my installs.  I have a couple of other receivers to protect besides the RSPdx so I would like to be on the safe side.

Are you contemplating a more wideband version receiver protector that would work with the full range of the Broadband Active Antenna ?  I've found an RF limiter made here in the US that shows 500 kHz to 2 GHz but it doesn't appear to have the power handling capability as yours and limits from +10 dBm to +13 dBm.

Thanks for your help.


Roger Need
 

On Fri, Feb 4, 2022 at 03:09 PM, Mark Lucas wrote:
Are you contemplating a more wideband version receiver protector that would work with the full range of the Broadband Active Antenna ?  I've found an RF limiter made here in the US that shows 500 kHz to 2 GHz but it doesn't appear to have the power handling capability as yours and limits from +10 dBm to +13 dBm.

As you go up in frequency capacitance across the receiver input becomes more critical. Less than a pF at 2 GHz. is required.   In order to get low capacitance parts (like protection diodes) the physical size has to be reduced and this limits the power they can handle.  For example - SDRplay RSP products have diode protection across the antenna input terminals that have low capacitance but they can only handle up to 10 dBm of power and that is only for a few seconds.  PIN diodes can be used at higher frequencies  because they operate in a different fashion then conventional diodes for this application but they are not intended for lower frequencies like HF. 

Another consideration.  Designs like the CCW protection unit use saturating isolation transformers.  They can only operate effectively over a limited range.

In summary what you ask for is possible but I don't believe it can be made at a price that hobbyists will pay...
Roger


Chris Moulding
 

Thanks for the detailed reply, Roger.

It's interesting to look at it from the SDRPlay point of view as it explains why they have the +10 dBm power limitation.

The design of the protection circuit is holding up the return of the Broadband Active Antenna.

I've designed it to work with the 4.7 V from the bias tee of an SDRPlay receiver and the prototype works perfectly.

Before I relaunch the antenna I need to add a protection circuit just in case the user plugs it into a 12 V bias tee.

Suitable diodes that can handle the over-voltage and the power handling required such as the Transil diodes have a junction capacitance of 10 nF that varies with the applied voltage like a varactor diode so cannot be put directly across the RF output.

I'm now looking at using the diode in a series LC circuit resonant below the lowest operating frequency but it needs a lot of testing to confirm that it doesn't generate intermodulation products or have some unexpected resonance in the operating band.

The amount of time, effort and used or failed components in the workshop bin that I've already spent on this is far greater than the original design process for the antenna!

RF protection isn't just a case of slap some diodes on and hope for the best. I suspect that the RF limiter quoting 500 kHz to 2 GHz would fail the tests we put our designs through. The tests include IP3 measurements, high power transmitters fed into it, high DC voltages (1500 V) etc. If the frequency coverage they are quoting is correct then they are probably using similar or identical diodes to those already inside the SDRPlay receivers.

Regards,

Chris


Roger Need
 

Chris,

You are probably familiar with the following but here goes.....

If the max power input is restricted to only a few watts the products from Mini-Circuits might be of interest to you.  They come in surface mount, boxes and SMA coaxial connector versions.  The go up to several GHz and down to less than 1 MHz.  The surface mount part could go in a box with additional circuitry like DC blocking, GDT and a static bleed resistor. 

https://www.minicircuits.com/WebStore/Limiters.html

When using diode limiters I am also a fan of the old wheat grain lightbulb trick that has been used at high power contest stations for years. .  They have a low cold resistance and when excess power is applied the resistance rises. At high powers the bulb fails and you have an open circuit.  Some protection products use the bulbs that snap into a fuse holder for easy user replacement. 

Mark Lucas might find this unit might meet his wideband requirement if less maximum power input is acceptable.  They are about $50 USD.  They don't publish any IP3 info so this is an unknown... 

https://www.minicircuits.com/pdfs/VLM-33W-2W-S+.pdf

IP3 is a concern when using diode limiters and as you know testing requires RF signal generators with low harmonic/spurious levels. 

Roger


Mark Lucas
 

Thanks for the replies.  I'm limited to have adequate vertical or horizontal separation between the TX and RX antennas without some creative engineering.  Power drop comes in on the opposite end of the house and there are other obstacles in the rear.  At best I'll have around 7 feet of horizontal separation.

Roger Need your link on the MiniCircuits limiter reminded me I had something similar in the junk box -- 3 HP 5086-7283 RF limiters.  I'm not sure their condition.  They show 10 W max input.  I believe they do limit to +10dBm.  I've seen several versions of these with different connectors.  Perhaps these will work.

Thanks again for everyone's help.


Mark Lucas
 

New Receiver Protection Unit delivered today.  Thanks much.